The present invention relates to a method of producing holes in a workpiece using a laser beam.
For a plurality of applications, for example, for manufacturing injection nozzles for gasoline and diesel vehicle engines, the production of holes with a definite hole geometry in the longitudinal direction of the bore hole is desirable. In particular, producing conical bore holes having a cross-section increasing toward one end may be required.
Such conical holes may be produced using wire corrosion, in which the conical geometry of the bore hole is produced by vibration of the erosion wire. This method may, however, represent a problem for small bore hole diameters in the range below 150 xcexcm due to the minimum wire diameter required for stability reasons. However, the production of small bore holes having high precision may for example, be required for injection nozzles. Furthermore, the wire corrosion method is believed to have the disadvantage that the conical hole geometry is difficult to reproduce and that the method is relatively slow and therefore cost-intensive.
With the exemplary method according to the present invention for producing holes in a workpiece using a laser beam, a definite hole geometry of the bore hole in its longitudinal direction can be obtained by adjusting the beam geometry and/or beam parameters of the laser beam. The bore hole may have a conical shape, and the bore hole diameter may increase in the direction of the beam exit. The exemplary method according to the present invention is believed to have the advantage that, by adjusting the beam geometry and/or the beam parameters of the laser beam, very flexible hole geometries can be produced with great precision. Another advantage is believed to be that the exemplary method according to the present invention can also be used for very small hole diameters in the range of 100 xcexcm and less. In addition, the drilling method is believed to be considerably faster compared to known drilling methods, which may provide considerable savings when used in industrial production.
According to an exemplary method according to the present invention, the laser beam is focused on the workpiece using a lens, the focal distance of the lens, the beam diameter at the point where the beam enters the lens, the focal diameter, and the power density of the laser beam is selected so that a conical bore hole is obtained. The focal area of the laser beam may be on the workpiece surface at the entry of the hole. After the focal area, the laser beam widens again, whereby, if the power density is sufficient, a conical longitudinal bore hole profile is obtained.
In order to obtain a very narrow laser beam bundle in the focal area, it may be expedient to widen the cross-section of the laser beam before it passes through the lens. A telescope-like lens arrangement which can be placed in the path of the beam almost anywhere between the laser and the lens can be used for this purpose.
According to another exemplary method according to the present invention, the laser beam performs a wobbling motion with respect to the workpiece, describing a conical surface. The longitudinal profile of the bore hole can be accurately defined through the wobbling motion. If the laser beam is also rotated about its own axis synchronously with the wobbling motion, the same point of the laser beam comes into contact with the workpiece in the azimuthal direction each time. Thereby an eccentric laser beam cross-section is equalized and very round hole cross examplary are obtained.
According to another variant of the method according to the present invention, the hole geometry in the longitudinal direction can be adjusted with the help of polarized laser light by selecting the direction of polarization and/or type of polarization. An oval hole geometry can be produced on the exit side of the laser beam by using a linearly polarized laser beam. Particularly round hole cross-sections can be achieved using circularly polarized laser light.
It is also possible to combine the wobbling motion of the laser beam with a definite polarization.
A device to according to an exemplary embodiment of the present invention for producing holes in a workpiece has a laser beam source, a lens for focusing the laser beam on the workpiece, a device for producing a wobbling motion of the laser beam, and a rotatable polarization device having a half-wave plate and a quarter-wave plate that can be rotated with respect to the half-wave plate.